arch/mips/kernel/sync-r4k.c

   1 /*
   2  * Count register synchronisation.
   3  *
   4  * All CPUs will have their count registers synchronised to the CPU0 next time
   5  * value. This can cause a small timewarp for CPU0. All other CPU's should
   6  * not have done anything significant (but they may have had interrupts
   7  * enabled briefly - prom_smp_finish() should not be responsible for enabling
   8  * interrupts...)
   9  *
  10  * FIXME: broken for SMTC
  11  */
  12
  13 #include <linux/kernel.h>
  14 #include <linux/init.h>
  15 #include <linux/irqflags.h>
  16 #include <linux/cpumask.h>
  17
  18 #include <asm/r4k-timer.h>
  19 #include <linux/atomic.h>
  20 #include <asm/barrier.h>
  21 #include <asm/mipsregs.h>
  22
  23 static atomic_t count_start_flag = ATOMIC_INIT(0);
  24 static atomic_t count_count_start = ATOMIC_INIT(0);
  25 static atomic_t count_count_stop = ATOMIC_INIT(0);
  26 static atomic_t count_reference = ATOMIC_INIT(0);
  27
  28 #define COUNTON 100
  29 #define NR_LOOPS 5
  30
  31 void synchronise_count_master(int cpu)
  32 {
  33         int i;
  34         unsigned long flags;
  35         unsigned int initcount;
  36
  37 #ifdef CONFIG_MIPS_MT_SMTC
  38         /*
  39          * SMTC needs to synchronise per VPE, not per CPU
  40          * ignore for now
  41          */
  42         return;
  43 #endif
  44
  45         printk(KERN_INFO "Synchronize counters for CPU %u: ", cpu);
  46
  47         local_irq_save(flags);
  48
  49         /*
  50          * Notify the slaves that it's time to start
  51          */
  52         atomic_set(&count_reference, read_c0_count());
  53         atomic_set(&count_start_flag, cpu);
  54         smp_wmb();
  55
  56         /* Count will be initialised to current timer for all CPU's */
  57         initcount = read_c0_count();
  58
  59         /*
  60          * We loop a few times to get a primed instruction cache,
  61          * then the last pass is more or less synchronised and
  62          * the master and slaves each set their cycle counters to a known
  63          * value all at once. This reduces the chance of having random offsets
  64          * between the processors, and guarantees that the maximum
  65          * delay between the cycle counters is never bigger than
  66          * the latency of information-passing (cachelines) between
  67          * two CPUs.
  68          */
  69
  70         for (i = 0; i < NR_LOOPS; i++) {
  71                 /* slaves loop on '!= 2' */
  72                 while (atomic_read(&count_count_start) != 1)
  73                         mb();
  74                 atomic_set(&count_count_stop, 0);
  75                 smp_wmb();
  76
  77                 /* this lets the slaves write their count register */
  78                 atomic_inc(&count_count_start);
  79
  80                 /*
  81                  * Everyone initialises count in the last loop:
  82                  */
  83                 if (i == NR_LOOPS-1)
  84                         write_c0_count(initcount);
  85
  86                 /*
  87                  * Wait for all slaves to leave the synchronization point:
  88                  */
  89                 while (atomic_read(&count_count_stop) != 1)
  90                         mb();
  91                 atomic_set(&count_count_start, 0);
  92                 smp_wmb();
  93                 atomic_inc(&count_count_stop);
  94         }
  95         /* Arrange for an interrupt in a short while */
  96         write_c0_compare(read_c0_count() + COUNTON);
  97         atomic_set(&count_start_flag, 0);
  98
  99         local_irq_restore(flags);
 100
 101         /*
 102          * i386 code reported the skew here, but the
 103          * count registers were almost certainly out of sync
 104          * so no point in alarming people
 105          */
 106         printk("done.\n");
 107 }
 108
 109 void synchronise_count_slave(int cpu)
 110 {
 111         int i;
 112         unsigned int initcount;
 113
 114 #ifdef CONFIG_MIPS_MT_SMTC
 115         /*
 116          * SMTC needs to synchronise per VPE, not per CPU
 117          * ignore for now
 118          */
 119         return;
 120 #endif
 121
 122         /*
 123          * Not every cpu is online at the time this gets called,
 124          * so we first wait for the master to say everyone is ready
 125          */
 126
 127         while (atomic_read(&count_start_flag) != cpu)
 128                 mb();
 129
 130         /* Count will be initialised to next expire for all CPU's */
 131         initcount = atomic_read(&count_reference);
 132
 133         for (i = 0; i < NR_LOOPS; i++) {
 134                 atomic_inc(&count_count_start);
 135                 while (atomic_read(&count_count_start) != 2)
 136                         mb();
 137
 138                 /*
 139                  * Everyone initialises count in the last loop:
 140                  */
 141                 if (i == NR_LOOPS-1)
 142                         write_c0_count(initcount);
 143
 144                 atomic_inc(&count_count_stop);
 145                 while (atomic_read(&count_count_stop) != 2)
 146                         mb();
 147         }
 148         /* Arrange for an interrupt in a short while */
 149         write_c0_compare(read_c0_count() + COUNTON);
 150 }
 151 #undef NR_LOOPS